1. Field of the Invention
The present invention relates to a fuel cell stack having a compact size and a light weight.
2. Description of the Related Art
A magnified sectional view illustrating major parts of a general fuel cell stack is shown in FIG. 5. The fuel cell stack 10 comprises a cell stack 13 in which a plurality of cell units 12 are electrically connected to one another in series and they are stacked in the horizontal direction as shown in FIG. 5.
The cell unit 12 comprises a unified body 20 which includes an electrolyte layer 18 positioned between an anode 14 and a cathode 16, and a pair of separators 22a, 22b which interpose the unified body 20 therebetween. In each of the both separators 22a, 22b, a first gas flow passage 24 for supplying and discharging the fuel gas (for example, hydrogen-containing gas containing a major component of hydrogen) with respect to the anode 14 is provided on the surface opposed to the anode 14, and a second gas flow passage 26 for supplying and discharging the oxygen-containing gas (for example, gas containing oxygen) with respect to the cathode 16 is provided on the surface opposed to the cathode 16. The unified body 20 is accommodated in an opening of a frame-shaped seal member 30.
Terminal electrodes 34a, 34b are electrically connected to the cell units 12, 12 which are positioned at both ends of the cell stack 13. Further, end plates 38a, 38b are arranged outside of the terminal electrodes 34a, 34b with electric leakage-preventive insulating plates 36a, 36b intervening therebetween respectively. Backup plates 40a, 40b are arranged outside of the respective end plates 38a, 38b respectively. A plurality of coned disc springs 42 are arranged between the end plate 38a and the backup plate 40a. 
A plurality of through-holes 44, which extends from one backup plate 40a to the other backup plate 40b, are formed through outer circumferential edges of the fuel cell stack 10. As shown in FIGS. 5 and 6, tie rods 46 are inserted into the through-holes 44 respectively. Both of the backup plates 40a, 40b are tightened by nuts 48 screw-engaged with the tie rods 46 (see FIG. 5), and thus the cell stack 13, the terminal electrodes 34a, 34b, and the end plates 38a, 38b are tightened. Accordingly, the coned disc springs 42 are compressed.
A fuel gas supply/discharge mechanism is connected to a first gas inlet passage 62 (see FIG. 6) and a first gas outlet passage 64 of the fuel cell stack 10. On the other hand, an oxygen-containing gas supply/discharge mechanism is connected to a second gas inlet passage 66 and a second gas outlet passage 68. A cooling water supply/discharge mechanism is connected to a cooling water inlet passage 70 and cooling water outlet passage 71 respectively. In FIG. 6, reference numeral 74 indicates mounting boss sections which are provided with through-holes (not shown) for inserting bolts (connecting members) to connect the fuel cell stack 10 to an automobile body.
When the fuel cell stack 10 as described above is operated, then the hydrogen-containing gas is supplied to the anodes 14, and the oxygen-containing gas such as air is supplied to the cathodes 16, while allowing the cooling water to flow through the fuel cell stack 10. The hydrogen in the hydrogen-containing gas is ionized on the anode 14 as represented by the following reaction formula (A). As a result, hydrogen ion and electron are generated.2H2→4H++4e  (A)
The hydrogen ion is moved via the electrolyte layer 18 to the cathode 16. The electron is extracted by an external circuit which is electrically connected to the anode 14 and the cathode 16. The electron is utilized as DC electric energy for energizing the external circuit.
Subsequently, the electron arrives at the cathode 16. The electron causes the reaction represented by the following reaction formula (B) together with the hydrogen ion moved to the cathode 16 and the oxygen in the oxygen-containing gas supplied to the cathode 16. Thus, water is generated.O2+4H++4e→2H2O  (B)
The operating fuel cell stack 10 which is thermally expanded compresses or elongates the coned disc springs 42 depending upon the amount of thermal expansion. Accordingly, the tightening force exerted on the cell stack 13 is maintained substantially equivalently in the thermally expanded fuel cell stack 10 as well.
The fuel cell stack 10, in which the backup plates 40a, 40b are mutually tightened with the tie rods 46 as described above, has a large external size, for the following reason. That is, it is necessary to provide any hole formation margin S for forming the through-hole 44 in order to allow the tie rod 46 to pass therethrough (see FIG. 6).
It is necessary that the tightening forces of the respective tie rods 46 are equivalent. The operating fuel cell stack 10 which is thermally expanded may lower the tightening force at a portion at which the tie rod 46 is loosely tightened as compared with other portions. As a result, any contact failure occurs in the cell stack 13 and the internal resistance is increased, deteriorating the power generation characteristics of the fuel cell stack 10 in some cases. Therefore, the thick backup plates 40a, 40b are used so that the large tightening force of the tie rods 46 may not cause any flexion. However, the thick backup plates 40a, 40b increase the size of the fuel cell stack 10 in the stacking direction. In other words, the external size of the fuel cell stack 10 is increased. For this reason, the weight of the fuel cell stack 10 is increased as well. Therefore, it is necessary to apply a large driving force for driving the automobile carrying the fuel cell stack 10.
A structure for holding the fuel cell stack without using any tie rod is known, in which the fuel cell stack is accommodated, for example, in an accommodating case or a stack, as described in Japanese Laid-Open Patent Publication Nos. 7-249426, 7-335243, and 9-92324. Japanese Laid-Open Patent Publication No. 2000-48850 suggests that two pressure plates are connected to one another at each of their four corner portions respectively with a holding member having a substantially L-shaped cross section.
Although a compact size can be realized for the fuel cell stack in any one of the foregoing cases, it is difficult to pressurize the cell units with an equivalent tightening force. The operating fuel cell stack which is thermally expanded increases the internal resistance of the fuel cell stack.
For example, when the amount of thermal expansion of the cell stack is large as compared with the holding member, the stack container, or the accommodating case having high rigidity, the thermal expansion of the cell stack is suppressed. As a result, an extremely large thermal stress is applied to the cell stack. Then, the constitutive member of the fuel cell stack is finally deformed in some cases. Consequently, the contact failure occurs in the cell stack, increasing the internal resistance of the fuel cell stack.